1. Field of the Invention
The present invention relates to an image heating apparatus used in an image forming apparatus such as a copying machine, a printer and the like.
2. Related Background Art
In the past, as image heating apparatuses used as fixing apparatuses, for example, there has been proposed a fixing apparatus comprising a heat generating resistor for generating heat when it is energized by a commercial power source, a temperature detector for detecting a temperature of the heat generating resistor, and a control means for controlling a transmission and interruption of an electric power from the commercial power source to the heat generating resistor on the basis of the temperature detected by the temperature detector, and such a fixing apparatus has been put to practical use.
In such a fixing apparatus, the temperature of the heat generating resistor is controlled by switching a condition of the apparatus to an energized condition or a de-energized condition for each half wave of an electric power outputted from the commercial power source, in accordance with the detected temperature from the temperature detector.
In this case, the control means controls the number of energizating waves within a predetermined period (control wave number) including a predetermined number of half waves of alternating current supplied from the commercial power source to the heat generating resistor, on the basis of the detected temperature.
However, since the predetermined period is constant, for example, if the predetermined period is long, although fine control of electric power can be made because the number of half waves (for each predetermined period) controllable for the transmission and interruption of the electric power from the commercial power source to the heat generating resistor is great, response to the detected temperature of the temperature detector is worsened because a time period between a previous control and a next control becomes long. On the other hand, if the predetermined period is short, although the response to the detected temperature of the temperature detector can be improved because the time period between the previous control and the next control is short, the fine control of electric power cannot be made because the number of half waves (for each predetermined period) controllable for the transmission and interruption of the electric power from the commercial power source to the heat generating resistor is small.
Therefore, when the predetermined period is constant, since the fine control of electric power and the response are not compatible, it is difficult to achieve further improvement in temperature control accuracy.